The wonderful observation of Kyoto school (see for instance [1]}) is the following relation
\(s_{(\alpha |\beta )}(\mathbf {p}^{a})= (-1)^{\sum _{i=1}^k \beta _i}\langle 0|e^{\sum _{k>0} \frac{1}{k} p^{(a)}_k J_k^{(a)}}\sum _{i=1}^k \psi _{\alpha _i}^{(a)}\psi ^{\dag (a)}_{-\beta _i-1} |0\rangle ,\quad \) 
